Speed reversion is an alternative pitch mode providing limited protection from abnormal low speed. It is normally engaged automatically from another mode. It is not available from GS.
GS mode is a AP ‘path on elevator’ mode, where the AT controls the selected speed datum. A speed reversion would require AP ‘speed on elevator’ mode, where a climb or descent is determined by the thrust setting. Reversion from GS would require a high thrust setting to ensure a climb. This logic is already built into the TOGA mode which provides a pitch up and an AP ‘speed on elevator’ climb, commanding the appropriate thrust setting on the AT.
Speed reversion is not an Airbus 'alpha floor' type of function, which is a higher order safety protection and operates independently of the AP/AT.
If someone wanted to end the thread on a high note,this would be it.Clear,concise explanation.